1. Field of the Invention
The invention relates to a process for treating washing water from a gas washing operation of an iron ore reduction plant.
2. Description of the Prior Art
From German Printed Publication 40 32 288 C2, a process for treating washing water from a gas washing operation of an iron ore reduction plant has become known, with which process the washing water, in one or several gas washers, is brought into direct contact with the crude gas, is withdrawn from the gas washer, and, after a separation of solids, is fed again to the gas washer in cooled-off form. To this end, the washing water largely freed from solids in a first thickener is, via a warm water basin, led into a cooling tower in which the water is cooled and separation of dissolved carbon dioxide, enrichment with oxygen, conversion of the calcium and magnesium hydrogen-carbonates into carbonates and of the iron bicarbonate as well as the iron sulphate into iron(III)hydroxide are carried out. Then, such chemically destabilized washing water will, in a second series connected thickener and freed from the newly formed and residual solids by an addition of flocculants and coagulants, be chemically stabilized and, via a cold water basin, fed back to the gas washer.
Gas from an iron ore reduction plant contains large quantities of freshly reduced, very reactive iron particles. A contact with the carbon dioxide-saturated washing water will give rise to iron bicarbonate. The iron particles also react, on sulphate ions, to become iron sulphate. In this way, a large part of the solid iron particles is converted to a soluble form. By expulsion of carbon dioxide and enrichment of the washing water with oxygen in the cooling tower, iron bicarbonate and iron sulphate will give rise to iron(III)hydroxide which is insoluble in water and which precipitates in the form of flocs. The precipitated iron(III)hydroxide flocs furthermore react on the oxygen present in the water and form solid-state iron oxides. In case of a melt reduction plant, the gas additionally contains a large quantity of calcium and magnesium oxide particles which, with the carbon dioxide-containing washing water, form bicarbonates that, after aeration in the cooling tower, precipitate as carbonates.
Such water having lost its chemical equilibrium and showing strong deposition tendencies is, from the cooling tower, fed to the second thickener. Due to an increased residence time, the solid precipitating procedures are terminated in the second thickener and the water is stabilized before it will again be used for a gas washing operation. In this way, the deposition problems in the critical and cleaning-problematic washer and operative area which is operated under elevated pressure of combustible and toxic gases are successfully solved. In the cooling tower area which, for cleaning and maintenance, is periodically turned off cell-by-cell, the solid depositions, however, are considerable. Already after a relatively short period of time, the water distributing systems and the packing of the cooling tower become fully overgrown and the depositions become so hard that the packing must be renewed several times a year. Instead of standard cooling towers which are used for the cooling of washing water from comparable gas washing operations, special cooling towers, thus, were developed so as to extend the time intervals between the cleaning cycles and to reduce the amount of maintenance. These special cooling towers are very big cooling towers with but a few built-ins and with cooling tower cups mounted at high level so as to enable discharging of the washing water towards the second thickener by free descent, with special nozzles for effecting irrigation of the water under a higher pressure, and including a variety of other measures so as to reduce the amount of cleaning and to shorten the times required for cleaning.
However, the amounts of cleaning and maintenance are still very great; furthermore, energy consumption is significantly higher than in case of a usual cooling tower and the specific investment costs for the cooling sawer likewise are very high. Moreover, space demand for two large thickeners, a big cooling tower, a pump station, and conduits is very high so that, in many cases, it is very problematic to accommodate the plant in the available space.
Another problem with that known process is the outward transfer of relatively large quantities of carbon monoxide into the atmosphere since the washing water in the gas washers which are operated under higher pressure comes into direct contact with the gases that, predominantly, are made up of carbon monoxide and carbon dioxide. Especially large gas quantities are, with the substantially colder backward-flow water from the second washing stage, into the atmosphere. Part of those gases evades in the first thickener which is operated under atmospheric pressure and the rest is expulsed in the cooling tower.